When this Folk singer performed a duet on his early album....?

Nashville Skyline, with a country music legend, he forgot the words to his own song. He was redirected back to the correct verse by that man in black who sang over him until the songwriter got the line right. The "blooper" was left on the album when it was released, the Folkie eschewing any retake, as was his usual habit in those days. What was the correct lyric?